The present invention relates to high-current cryogenic leads for pulsed power and high thermal inertia applications and, more particularly, cryogenic leads which are used in conjunction with supercooled coils to provide high thermal stability.
When electrical current flows between a current source which is at room temperature and a coil which is immersed in a pool of liquid refrigerant, such as helium, the electrical connections, or leads, between the source and the coil must be cooled to prevent overheating due to a high current flow therethrough. In order to provide sufficient cooling, cryogenic leads are typically provided with a plurality of channels through which a refrigerant can flow. One method for providing this refrigerant flow is to construct the cryogenic lead from a plurality of conductive tubes. This tubular arrangement is described in MFTF Magnet Cryostability, 8th Symposium on Engineering Problems of Fusion Research, 1979, IEEE Publication No. 79CH1441-5NPS pages 1761-1764 by J. H. VanSant. Each tube provides a parallel electrical path with its associated tubes and acts as a fluid conduit for refrigerant to flow through its central bore.
When a cryogenic lead is constructed from a plurality of tubes as described above, the current flowing through any individual tube must be generally equal to the current flowing through each of its associated tubes. Otherwise, if one tube conducts a significantly higher amount of current than the rest, it can potentially overheat and be damaged.
It should be understood that when a plurality of tubes are conducting both refrigerant flow and electric current in parallel, slight variations of temperature, rate of coolant flow, refrigerant viscosity and pressure drop along the length of the tube can exist between individual tubes in a particular cryogenic lead. These slight variations between the tubes of a cryogenic lead can cause the physical conditions of two or more distinct tubes to diverge and, therefore, cause the cryogenic lead to be thermally unstable.
Potential thermal instability is caused by the functional relationships which exist between refrigerant flow rate, tube temperature, refrigerant viscosity and pressure drop from one end of a tube to the other. For example, using a two-tube cryogenic lead as an example, one tube may experience a temporarily increased flow rate through it. This increased flow rate of a refrigerant would temporarily lower the temperature of that tube and the coolant relative to its other associated tube and coolant flowing therethrough. When the temperature of the refrigerant flowing through that tube is lowered, its viscosity is reduced. This lower viscosity causes the pressure drop along the length of that tube to be reduced which, in turn, causes a higher flow rate of the available refrigerant flowing through that cooler tube. This increased flow rate causes more efficient cooling, which further lowers the viscosity and pressure drop. This induces a reduced coolant flow in the associated tube. It should be apparent that these functional relationships, following a perturbation of refrigerant flow through a specific tube, cause the thermal conditions within the exemplary two-tube lead to become unstable resulting in one tube's temperature being significantly reduced in comparison to its initial condition and producing a temperature rise in the associated tube in which no perturbation occurred. It is the solution of this thermal instability problem to which the present invention is directed.
A cryogenic lead made in accordance with the present invention comprises a plurality of tubes connected both electrically and fluidly in parallel. Each individual tube is connected electrically to a conductive block at each of its ends. Each tube is a laminar composite of two coaxial and concentric tubular sleeves. The inner tubular sleeve is made of copper and the outer tubular sleeve is made of stainless steel. The inner copper sleeve provides good electrical conductivity and the outer stainless steel sleeve acts as a thermal mass into which heat can be transferred from the copper sleeve to prevent overheating damage to the inner conductive copper sleeve as might occur with a momentary loss of coolant flow. In order to prevent the thermal instability described above, each tube is provided with one or more fluid flow restrictions placed in its inner bore. Each of these restrictions has an orifice which restricts and reduces the flow of coolant gas through the tube. These orifices also reduce the amount of radiated heat that can be transmitted through the tube in the direction opposite that of the refrigerant flow. In order to further reduce this radiation, the orifices in the above-described restrictions can be arranged in such a way so that they are not aligned.
The present invention operates on the concept that if the functional relationship between changes in the viscosity and the pressure drop along the length of the tube can be reduced or essentially eliminated, the abovedescribed thermal instability can be prevented. The pressure drop along a conduit through which a fluid is flowing is a function of two independent factors. The first, which is related to the frictional effects of the conduit on the fluid flow, includes a direct relationship with the viscosity of the fluid and the length of the conduit and an inverse relationship with the velocity of the fluid and the square of the diameter of the conduit. The second, which is related to obstructions in the conduit which create turbulent flow, is a loss coefficient which is directly related to the pressure drop along the length of the tube and is a function of the dimensions and configuration of the obstruction causing the turbulent flow. In a tube with no obstructions, the first factor dominates and a reduction in the viscosity of the fluid produces a corresponding reduction in the pressure drop which cooperates to exacerbate the thermal instability as described above. The present invention utilizes one or more orifices placed within the bore of the conduit in order to increase the effect of the second factor, discussed above, on the pressure drop within the tube. If this second factor is made significant enough to dominate the effects on pressure drop, relative to the viscosity effects, the tube's tendency towards thermal instability can be reduced to insignificance.
The inclusion of a predetermined number of orifices, or fluid flow restrictors, within the conduit therefore can eliminate thermal instability in the cryogenic lead and also increase the lead's thermal efficiency by reducing the amount of heat radiated through the tubes in a direction opposite to that of the coolant flow. It should, therefore, be apparent that the present invention provides a cryogenic lead construction which prevents thermal instability while increasing the thermal efficiency of the lead's operation.